Capability to create a conversation on someone else's iPad from a different location?

I found a conversation on my fiance's iMessage account between him and another girl—a very inappropriate one. It was the only conversation besides the one he was having with me marked the same time and the same date. He tells me that she had an app that created the conversation, that it was fake, and she somehow got it to save on his iPad to his iMessages

I get that maybe she could log in on his account but wouldn't he also see the text as they were "created" if he was texting me simultaneously?